The present invention relates to optical communications network systems that carry wavelength division multiplexed (WDM) signals. More particularly, the present invention relates to WDM optical communications network systems that utilize optical add-drop multiplexers, wherein each such multiplexers add and/or remove one or more optical signals from the network.
Optical add-drop multiplexer (OADM) technology substantially reduces the cost of Dense Wavelength Division Multiplexing (DWDM) optical networks. An example of a conventional OADM configuration within a DWDM system is shown in FIG. 1. In the conventional system 100 shown in FIG. 1, a multi-channel optical signal 102 is delivered to the input port 103 of OADM device 120. The optical signal comprises a plurality of channels, each comprising a different unique wavelength range, where each channel is denoted by its respective center wavelength, xcex1, xcex2, xcex3, etc. A first optical filter 106a is used to remove or xe2x80x9cdropxe2x80x9d one of the incoming multiple channels, e.g., xcex1, and to pass through the remaining xe2x80x9cexpressxe2x80x9d channels xcex2, xcex3, and xcex4 as signals 104. A second optical filter 106b is used to add a channel xcex1xe2x80x2 into the optical path containing the express channels. The express channels exit from OADM 120 apparatus together with the added channel xcex1xe2x80x2 as a single combined signal 112 at the output port 105.
An example of an optical communications network system containing a conventional OADM is demonstrated in FIG. 2. In the conventional network system 200, a plurality of channels xcex1, xcex2, . . . , xcexN are transmitted between end locations 202a and 202b. The optical network comprises end locations 202a and 202b, a plurality of n intermediate locations, or xe2x80x9cnodesxe2x80x9d 206.1-206.n disposed between the end locations, and a sequence of optical fiber spans 208.1-208.(n+1) optically connecting the nodes 206.1-206.n and the end locations 202a-202b to one another in a single chain. The first end location 202a comprises a WDM multiplexer (MUX) 204a that combines the channels from separate input paths into a single combined signal that is delivered to a first span 208.1 of optical fiber. Likewise, the second end location 202b comprises a WDM de-multiplexer (DEMUX) 204b that receives a set of combined channels from the last span 208.(n+1) of optical fiber and separates these channels into separate output paths.
Optical signals xcexi, xcexj, xcexixe2x80x2, and xcexjxe2x80x2 are added and/or dropped from the chain of optical fiber spans 208.1-208.(n+1) at each of the nodes 206.1-206.n. Each node is disposed between two such consecutive spans of optical fiber and the optical fiber spans join the nodes to one another. Each of the nodes 206.1-206.n comprises a respective one of a set of n OADMs 210.1-210.n that performs the adding and dropping of channels at the node. The OADMs are required in order to allow each of the nodes access to a respective portion of the network traffic while, at the same time maintain the integrity of other channels. Without such OADMs, all channels would have to be terminated at each intermediate node even for a small portion of traffic exchange.
One characteristic of the conventional OADM structure shown in FIG. 1 is that the added channel xcexxe2x80x21 generally comprises an optical power that is different from the powers of the express channels. This power difference arises because the added channel originates from a different optical path from those of the express channels and thus generally incurs a unique set of insertion loss along this path. This unequal-power characteristic does not impose any negative impact to the early local (e.g., xe2x80x9cmetropolitanxe2x80x9d or xe2x80x9cmetroxe2x80x9d) multi-channel OADM systems wherein no optical amplifiers are used. However, the trend of late is to widely deploy amplifiers in such metro OADM systems in order to stretch the link distance and reach more customers. If channels in such an optical network have differing power levels, the weak signals could quickly dissipate after passing through a chain of amplifiers, due to the gain competition of the amplifiers. Therefore, the use of conventional OADM apparatus within a metro optical network also comprising optical amplifiers presents some problems.
In order to overcome the aforementioned problems with conventional OADMs, and particularly, systems such as metro systems making use of such OADMs, the present invention discloses a novel optical network system that utilizes a new inventive design of self-adjusting OADM. The self-adjusting OADM in accordance with the invention automatically adjusts the power of an added channel with reference to the power of a dropped channel, so that the output channels of the OADM all have a comparable power level. The self-adjusting OADM of the present invention comprises a first optical filter or similar wavelength-selective component that removes a channel from a WDM (a wavelength division multiplexed) signal and passes through the other channels, a drop line that receives the removed channel, a beam combiner, such as a second optical filter, that adds a new channel to the other channels, an add line that delivers the added channel to the network, a first optical tap on the drop line, a second optical tap on the add line, a variable optical attenuator (VOA) on the add line, and a controller. The first and second optical taps divert known sample portions of the dropped and added channels, respectively, to the controller. The controller receives the sample portions, and, based upon these samples, controls the variable optical attenuator so that the added channel enters into the network at a suitable power level, preferably one that is substantially equal to those of the other channels.
The invention further comprises an optical ring network having a plurality of optical exchange nodes and a plurality of optical amplifiers disposed in a ring configuration, wherein each node includes a self-adjusting OADM. In accordance with one embodiment, the optical ring network is a two-fiber ring network having a pair of rings, one for signals propagating in what is referred to as a clockwise direction, and one for signals propagating in a counterclockwise direction. Each node includes a switching assembly which comprises a pair of OADMs each in optical communication with an associated one of the rings. The switching assembly is also provided with an optical switch which operates in one of two states in accordance with a first control signal from a controller also provided in the switching assembly. In one state, the optical switch propagates signals from one of the OADMs and associated ring to an optical receiver at a subscriber location, while in the other state the optical switch propagates signals from the other one of the OADMs and associated ring to the optical receiver. The first control signal is provided to the switch by a controller which outputs the control signal in accordance with the relative power of drop signals from the two OADMs. In accordance with a further aspect of the invention, the controller also issues second and third control signals, which are used to control the attenuation of an add signal provided by an optical transmitter to the OADMs for combination with the WDM signals in the two rings of the two-fiber ring network. The second and third control signals control the attenuation of first and second variable optical attenuators operating on the add signal. The self-adjusting OADM of the present invention enables the deployment of optical amplifiers within metropolitan optical network so that the link distance is significantly increased.
The present invention includes a method for controlling the power of an add channel of an optical add-drop multiplexer (OADM) used to add the add channel to a wavelength division multiplexed (WDM) signal and to drop a drop channel from the WDM signal, the method comprising comparing the power of the add channel with the power of the drop channel and attenuating the power of the add channel to match the power of the express WDM channels.
The present invention also includes a method for selectively directing, to a receiver, the optical signal of a predetermined channel of a wavelength division multiplexed (WDM) signal of a two-fiber ring, the two-fiber ring having a first ring and a second ring each adapted to propagate said predetermined channel, the method comprising comparing at least a portion of the power of the optical signal of the predetermined channel of the first ring with at least a portion of the power of the optical signal of the predetermined channel of the second ring, and directing the signal of the predetermined channel having the greater power to the to the receiver.
The present invention also includes a method for conducting a wavelength division multiplexed (WDM) signal between an interconnection line and one or more subscribers using a fiber ring network, the WDM signal comprising a plurality optical signals each associated with a channel, the method comprising comparing the power of a predetermined channel of the WDM signal from the fiber network with the power of an add channel from a subscriber, attenuating the power the of the add channel as necessary to match the power of the predetermined channel, and combining the add channel with the WDM signal.